These experiments were initiated in the Weizman Institute and are being continued at the N.I.H. Genetic and Biochemical evidence obtained in several laboratories has defined the product of the SV40 A gene, the T-antigen, as a protein which is involved in viral DNA synthesis and RNA transcription. A specific interaction of this protein with the DNA sequences on the viral genome where these processes originate (termed the Origin) has been established. We have undertaken an analysis of T-antigen in different forms with respect to its functional association with DNA. The data suggest that newly synthesized T-antigen may be modified in the infected cell before it will bind specifically to the origin. Either during, after, or instead of its specific interaction with DNA, the T-antigen forms oligomers which are no longer active, and which represent the majority of the T-antigen in the cell.